add_action('user_register', 'my_on_user_register');
function my_on_user_register($user_id){

    $args = array(
        'length'    => 6,
        'before'    => date("Y")
    );
    $random_number = my_random_string($args);
    update_user_meta($user_id, 'random_number', $random_number);

}

add_action('show_user_profile', 'my_extra_user_profile_fields');
add_action('edit_user_profile', 'my_extra_user_profile_fields');
function my_extra_user_profile_fields($user){

    $random_number = get_the_author_meta('random_number', $user->ID);
?>
    <h3><?php _e('Custom Properties'); ?></h3>

    <table class="form-table">
        <tr>
            <th><label for="address"><?php _e('Random Number'); ?></label></th>
            <td><?php echo $random_number; ?></td>
        </tr>
    </table>
<?php
}